(Not Applicable)
(Need info from inventor?)
The present invention relates generally to a control system for an aircraft and, more particularly, a control system for alleviating a gust load on an aircraft wing.
Gust load alleviation has been a major problem for aircraft designers. Gust loads are created on a wing when an aircraft encounters an upward or downward moving pocket of air, more commonly referred to as turbulence. These sudden wind gusts induce very rapid changes in the wing angle of attack, which translates into sudden increases in the wing lift. The increase in wing lift corresponds to a sudden escalation in the bending moment on the wing structure that can exceed its load-carrying capability. In the absence of a control system for alleviating gust loads, the wing structure must be strengthened to accommodate the increased bending moment. This strengthening means higher structural weight and decreased aircraft performance. However, reducing or alleviating the gust load corresponds to a reduction in the bending moment on the wing. If gust loads can be alleviated to levels at or below maneuver load thresholds, aircraft weight can be substantially reduced, resulting in improved aircraft performance such as an increase in range or payload.
It is common for aircraft with stiff, shorter span wings to utilize conventional trailing edge surfaces or spoilers for gust load alleviation. For example, on stiff wings, ailerons can be placed near the wing tip both for roll control and for alleviating gust loads. Ailerons placed at this location on stiff wings are less prone to loss of effectiveness due to wing twisting. In addition, ailerons on stiff, shorter wings can extend across a significant portion of the wing span, adding to their effectiveness. By actuating the ailerons in a symmetrical manner in the trailing edge up position, the lift on the wing is reduced and can even allow for a net downward load on the portion of the wing spanned by the aileron, thereby alleviating the effects of a gust load. However, on long endurance aircraft which typically utilize long, slender wings, ailerons typically cannot be placed at the wing tip for alleviating gust loads. This is because the wing will deform in a twisting manner when the aileron is activated, minimizing or negating the effectiveness. Long, slender wings are typically very flexible due to the long span and the need to keep the aircraft weight to a minimum. For this reason, on slender-winged aircraft, ailerons are typically placed farther inboard, minimizing their effectiveness as load alleviation devices. Thus, there is a need for a gust load alleviation device that can be utilized on aircraft with long, slender wings.
The prior art approaches to alleviating wing gust loads on an aircraft wing are numerous. The prior art describes a flight control system for an aircraft including trailing edge airfoil members pivotally mounted at the trailing edge of each wing of an aircraft and selectively movable on a laterally extending axis between raised and lowered positions for imparting rolling motion to the aircraft. Leading edge airfoil members mounted adjacent the leading edge are transversely movable between retracted positions and an extended position protruding from the leading edge for imparting countervailing aerodynamic forces on the wings to counteract the effects of aeroelastic wing deformation caused by the trailing edge airfoil members, increasing the angle of attack of the wing. A pair of actuators are mounted on a wing box member at laterally spaced locations for retracting and extending the leading edge airfoil member. The actuator mechanisms are simultaneously operable such that they are capable of extending the leading edge airfoil members with or without rotation of the leading edge airfoil in either an up or down position relative to the wing. The advantage of this flight control system is that it imparts countervailing aerodynamic forces on the wings to counteract the effects of aeroelastic wing twisting which can occur when a deflected trailing edge airfoil increases the angle of attach of the wing. The drawback of this flight control system is that it is only designed to provide leading edge up twisting to counteract the adverse effect of down twisting of an aileron, the down twisting of an aileron decreasing the aileron""s roll effectiveness. In high aspect-ratio wings, up gusts such as the upward thermal drafts more commonly referred to as turbulence in commercial airliners, are more critical to the load-carrying capability of a wing than down gusts. A device that can twist a wing down is more critical in the loading capability of an aircraft than a device that can twist a wing up because the wings are already supporting at least one-G loading in order for the aircraft to maintain level flight. Although it is advantageous to have the capability to twist a wing up in order to alleviate down gusts, a device that can twist a wing down may be the only wing twisting mechanism necessary for an aircraft.
The prior art also describes a system for controlling aeroelastic deflection of the wings beyond control surface reversal. Control surface reversal occurs when high dynamic pressure on trailing edge control surfaces of a wing cause the wing to twist in the area of the control surface. Normally, differential control surface deflections cause differential lift of the opposing wings on either side of the fuselage. This differential in lift causes the aircraft to roll about its longitudinal axis, initiating a turn of the aircraft. However, at high dynamic pressure on the control surfaces, instead of causing differential lift on the opposing wings, the wing will twist so that the aircraft roll command is minimized or even negated. The system for controlling aeroelastic deflection includes flexible wings, leading and trailing edge control surfaces, a large, distributed sensor network to measure selected aircraft flight parameters, an information processing system to receive and process pilot command signals and signals from the sensors, and control mechanisms in the wing that respond to processed signals from the information processing system. The control mechanisms selectively position the control surfaces to produce loads such that the wings are deflected in a desired manner for aircraft control. The system can be used for aircraft control, maneuver performance and gust load alleviation. The advantage of this system is that its large sensor network is integrated and linked to a central flight computer so that the flight performance data is utilized in a multifunctional manner to optimize control surfaces based on both pilot commands, preprogrammed flight data such as drag and load minimizing criterion, and uncommanded flight events such as control surface flutter and gust loading events. However, the drawbacks of this system are that it is not retrofitable to existing aircraft, it is not self-contained in that it cannot be installed as a separate system at the aircraft wing tip, and it is dependent on a centralized flight control computer for actuation.
As can be seen, there is a need for a device for alleviating gust loads on an aircraft wing that can twist a wing both down and up or a mixture of both. Additionally, there is a need for a device for alleviating gust loads that is self-contained and that may be retrofittable into existing aircraft. Furthermore, there is a need for a device for alleviating gust loads on an aircraft wing that is low cost and of simple construction with a minimal number of parts. Finally, there is a need for a device for alleviating gust loads on an aircraft wing that may operate either independently of or in conjunction with a flight control computer.
The present invention specifically addresses and alleviates the above referenced deficiencies associated with the use of gust load alleviating devices of the prior art. More particularly, the present invention is an improved gust load alleviating device that exhibits improved operability compared to devices of the prior art in that it is capable of reducing wing bending moment by twisting a wing both down and up or a mixture of both in response to gust loads. Furthermore, the gust load alleviation device is adaptable to aircraft having long, slender wings and may be retrofittable into existing aircraft.
In accordance with an embodiment of the present invention, there is provided a control system for alleviating a gust load on an aircraft wing. The control system is incorporated into an aircraft wherein a deflector mechanism acts to twist the wing in order to effect an overall reduction in the bending moment of the wing. The effect of wing twisting by the deflector mechanism is performed in conjunction with a vertical motion sensor, a sensor signal processor and a deflector controller. The vertical motion sensor measures the vertical motion of the wing tip in response to a gust load on the wing and generates a sensor output signal. A sensor signal processor generates a deflector control signal in response to the sensor output signal. A deflector controller regulates the deflector mechanism movement in response to the deflector control signal such that the deflector mechanism is alternately deployed and retracted into and out of the airstream.
In operation, during the course of the flight of the aircraft, gust and maneuver loads acting on the aircraft wing induce vertical motion of the wing tip. The vertical motion sensor senses the vertical motion of the wing tip and generates a sensor output signal. Because the rate of change of vertical acceleration of the wing tip is very slow for the maneuver load as compared to the gust load, the vertical motion sensor distinguishes the maneuver load from the gust load and only generates a sensor output signal in response to the gust load, and not in response to the maneuver load. The signal sensor processor estimates the vertical motion of the wing tip and compares the wing tip vertical motion to a preset threshold value representing the peak vertical motion due to a maneuver load. Such a maneuver load may be characterized as the load on the wing tip caused by control surface deflections during normal flight maneuvering, such as when turning the aircraft. After comparing the wing tip vertical motion to the preset threshold value, the signal sensor processor then generates a deflector control signal, but only if the wing tip vertical motion represented by the sensor output signal is greater than the threshold value.
If so, the generated deflector control signal is transmitted to the deflector controller, which regulates the deflector mechanism movement in response to the deflector control signal such that the deflector mechanism is alternately deployed and retracted into and out of the airstream at a duration and degree which is sufficient to counteract an increase in bending moment on the wing due to the gust load on the wing. The deflector controller deploys and retracts the deflector mechanism either partially or fully and for any length of time over the course of the flight of the aircraft. The deflector mechanism generates a deflector torque on the wing in response to an aerodynamic force of the airstream on the deflector mechanism. The deflector torque may effect either an upward or a downward twisting motion on the wing depending on whether the gust load effects an upward or a downward vertical motion of the wing tip. In response to a downward gust, an upward twisting of the wing increases the angle of attack of the wing locally, generating increased lift at that location which in turn increases the wing bending moment at the wing root. In response to an upward gust, a downward twisting of the wing decreases the angle of attack of the wing locally, generating decreased lift at that location which in turn decreases the wing bending moment at the wing root.
The present invention may be utilized on any aircraft wing as all aircraft wings are flexible to some degree. Furthermore, the control system may be adapted for use on aircraft with long slender wings as the smooth twisting of the wing from the wing tip to the wing root is efficient in alleviating gust loads on the wing.